The present invention relates to electromagnetically-induced thermoacoustic imaging and, more specifically, to real-time scanning electromagnetically-induced thermoacoustic imaging of biological tissues.
Electromagnetically-induced thermoacoustic imaging is based on the photoacoustic effect, i.e., the generation of acoustic waves by the deposition of short-pulse electromagnetic energy safely into biological tissues. The electromagnetic wave (microwave or radio frequency) for this technology is short-pulsed, and its power is within the IEEE safety limits. The electromagnetically-induced acoustic wave is detected with an ultrasonic detector or detector array for imaging. The contrast between tumors and normal tissues in the microwave regime is very good. Cancerous breast tissues, for example, are found to be 2-5 times more strongly absorbing than surrounding normal breast tissues in the microwave range, which has been attributed to an increase in bound water and sodium within malignant cells.
Purely-microwave imaging of biological tissues, however, is fundamentally limited to poor resolution (on the order of 10 mm) because of the larger wavelength of microwave. Also, purely-microwave imaging has had difficulties in multi-channel detection of microwave without cross coupling, in reconstruction algorithms, and especially in achieving good spatial resolution because of the strong diffraction of microwaves. Purely-ultrasound imaging (ultrasonography), an established medical imaging modality, can yield good spatial resolution, but has poor contrast for early-stage tumors. Electromagnetically-induced thermoacoustic imaging, and microwave-induced thermoacoustic imaging in particular, can potentially bridge the gap and fuse the advantages of the two imaging modalities.
If optical radiation instead of microwave radiation is used, this thermoacoustic phenomenon is better known as photoacoustics. Microwave-induced thermoacoustic imaging shares similar principles with its optical counterpart. However, microwave-induced thermoacoustic imaging may find unique applications in medical imaging because microwave radiation provides a deeper penetration depth in biological tissues than light and has different contrast mechanisms.
Microwave-induced thermoacoustics has been used to quantify physical parameters in media such as the power density and the concentration of a given substance. Several investigators have employed microwave-induced thermoacoustics in the 1980s for imaging of biological tissues. These early works, however, did not produce any tomographic or depth-resolved images. Recently, images of biological tissues have been computationally reconstructed based on microwave-induced thermoacoustics. This approach requires the measurement of a large amount of data around the tissue and post-processing computation.
X-ray mammography is the current standard clinical tool for breast cancer screening. Although effective, it has difficulties in imaging premenopausal breasts, and has the medical and environmental disadvantages attendant upon the use of ionizing radiation.
In accordance with the invention, methods and apparatus for real-time, non-invasive electromagnetically-induced thermoacoustic scanning of biological tissue are provided, in which the tissue to be imaged is irradiated with short pulses of electromagnetic energy and the resulting induced thermoacoustic waves are detected by one or more ultrasonic transducers to provide a time-domain signal. The time-domain signal is converted to a spatial one-dimensional image of the tissue along the transducer axis. By utilizing a one-dimensional array of transducer elements or by scanning the tissue in a direction transverse to the transducer axis and repeating the irradiating, detecting and signal-converting steps at spaced points along the transverse direction, a two-dimensional image of the tissue is provided. A three-dimensional image may be provided by repeating the foregoing steps in a second direction transverse to both the transducer axis and the first direction. In accordance with the invention, such two-dimensional and three-dimensional images are provided in real time without computational reconstruction of the image.
The applied electromagnetic energy field is preferably compressed to a narrow wave to minimize the exposed volume of tissue and to improve image quality. A tapered waveguide may be employed for that purpose. The acoustic axis of the focused ultrasonic transducer or of the transducer array, is located in the volume of the electromagnetic wave. A curved illumination interface may be provided between the waveguide and the tissue to enhance efficient energy transfer to the tissue and to focus the electromagnetic energy within the region of the tissue to be imaged. This may be accomplished by curving the tissue itself, or by curving the exit end of the waveguide and bending the tissue surface accordingly.
In an advantageous embodiment of the invention, the electromagnetic energy is in the microwave range and preferably within the range of from 300 MHz to 3 GHz. The microwave pulse width is preferably within the range of from 0.1 xcexcs to 0.5 xcexcs.
The ultrasonic transducer may comprise a single-element focused transducer or multiple single-element focused transducers forming an array. Alternatively, an array of unfocused transducers may be used, in which case synthetic focusing of the output signals is employed to generate the two-dimensional tomographic images.
For better visualization of deeper tissues, the transducer signals may be gain compensated to offset electromagnetic wave attenuation within the tissue.
The electromagnetically-induced thermoacoustic scanning technique of the invention is compatible with existing ultrasonographic equipment. In accordance with the invention, both thermoacoustic images and ultrasonograms may be recorded of the same sampling cross section or volume, thereby providing both types of images in real time for co-registration. The diagnostic information available to the physician is thus enhanced.
The advantages of the invention relative to prior imaging technologies include the use of non-ionizing radiation, enhanced imaging resolution, increased penetration depth, high contrast between tumors and normal tissues, real-time imaging, and co-registration between thermoacoustic images and ultrasonographic images.